$ oerr ora 12154
12154, 00000, "TNS:could not resolve the connect identifier specified"
// *Cause: A connection to a database or other service was requested using
// a connect identifier, and the connect identifier specified could not
// be resolved into a connect descriptor using one of the naming methods
// configured. For example, if the type of connect identifier used was a
// net service name then the net service name could not be found in a
// naming method repository, or the repository could not be
// located or reached.
// *Action:
// - If you are using local naming (TNSNAMES.ORA file):
// - Make sure that "TNSNAMES" is listed as one of the values of the
// NAMES.DIRECTORY_PATH parameter in the Oracle Net profile
// (SQLNET.ORA)
// - Verify that a TNSNAMES.ORA file exists and is in the proper
// directory and is accessible.
// - Check that the net service name used as the connect identifier
// exists in the TNSNAMES.ORA file.
// - Make sure there are no syntax errors anywhere in the TNSNAMES.ORA
// file. Look for unmatched parentheses or stray characters. Errors
// in a TNSNAMES.ORA file may make it unusable.
// - If you are using directory naming:
// - Verify that "LDAP" is listed as one of the values of the
// NAMES.DIRETORY_PATH parameter in the Oracle Net profile
// (SQLNET.ORA).
// - Verify that the LDAP directory server is up and that it is
// accessible.
// - Verify that the net service name or database name used as the
// connect identifier is configured in the directory.
// - Verify that the default context being used is correct by
// specifying a fully qualified net service name or a full LDAP DN
// as the connect identifier
// - If you are using easy connect naming:
// - Verify that "EZCONNECT" is listed as one of the values of the
// NAMES.DIRETORY_PATH parameter in the Oracle Net profile
// (SQLNET.ORA).
// - Make sure the host, port and service name specified
// are correct.
// - Try enclosing the connect identifier in quote marks.
//
// See the Oracle Net Services Administrators Guide or the Oracle
// operating system specific guide for more information on naming.
$

unless there is a TNS_ADMIN environment defined on your local system, Forms 6i uses the one residing under the network/admin/ subdirectory of the Forms 6i installation directory. So, you need to ensure that you are using the correct tnsnames.ora file. In that tnsnames.ora file, you need an entry for the XE similar to this:

Once the TNS_ADMIN and TWO_TASK are properly defined, then you should have no problem. We have been using Forms/Reports 6i based client/server application on Linux to connect to 817 to 11gR2 - it is the same OFA principle. Once Forms is installed it's own Home, and the above mentioned environment variables are set, you should be able to connect to your target database from your form login screen or developer window. On our system (Linux), I have a Forms 6i specific profile file, that I am sourcing prior to start the forms 6i developer. It might be helpful - so here it is.